Nowadays, there is a wide range of fields in which 3D mapping and information is applied to enhance different technologies: being video generation techniques including depth information, guiding of vehicles through mapping of the surface where the vehicle stands, object location, quality control in a manufacturing process, etc.
One of such 3D mapping used in said fields is the scanning of surfaces by means of systems based on Time of Flight measuring techniques (known as TOF), which have been widely used in the past for obtaining images with depth information.
Said systems based on TOF techniques commonly involve the implementation of a system for transmitting and receiving a light beam, in order to measure distances from a surface. The first imaging systems based on TOF used mechanical scanning techniques and comprised a mechanical system responsible for managing the transmission and reception of the light beam in a determined direction. In any case, the measure was based on a single point with a single sensor and the image was formed from a mechanical scanning and the correlation of “measurement” and “trigger point” of the light beam.
Furthermore, the mechanical scanning involved problems relating to the vibration of the components of the system, their lower durability and big size.
As opposed to the systems for receiving a light beam used in the described mechanical systems, there also exist systems for receiving a light beam based on arrays of light sensors.
Basically, there are scanning systems that comprise an array of light sensors of a specific size (n columns by m rows), which may receive and detect the reflection of the light beam on the surface all at once. These systems, known as Flash Ladars, use modulated and/or pulsed TOF measuring techniques, which are widely known. Furthermore, by using these systems to obtain a digital image, there is no need to perform a mechanical scanning of a surface in two dimensions, since the array itself defines a two-dimensional surface, which in the end will define the image size.
More specifically, as can be seen in FIG. 1, a system for scanning a surface usually used nowadays (in this example, a system using pulsed TOF techniques, although modulated ones are also used) comprises a laser or LED beam transmitter 1 that transmits a light beam 2 to the surface 3 to be scanned, this light beam being reflected on the surface and being received by the array of light sensors 4 acting as a receiving system and acting as a detector for determining the moment of receiving each portion of the light beam 2. Further, the system comprises a beam splitter 5 which directs a portion of the light beam to a detector 9 for determining the moment of starting the transmission of the light beam, the required optical elements 6,7, and a device 8 for counting the TOF values (taking into account the moment in which the detector 9 detects the portion of the light beam split by the beam splitter 5) for each portion of the light beam received by the array of light sensors 4 and for determining the distance between the transmitter 1 and the surface 3 taking into account the corresponding TOF value. This way, the system enables to obtain a 3D digital image of the scanned surface.
An example of a system using a pulsed TOF technique device developed by MIT is described in “Real-Time 3D Ladar Imaging” (Cho, Anderson, et. al., LINCOLN LABORATORY JOURNAL, volume 16, no. 1, 2006). Such a TOF device comprises an array of light sensors, and it uses pulsed signals. More specifically, a 32×32 pixel sensor is used, which is able to perform measurements at a frequency of up to 16 KHz, using a signal with a wavelength of 532 nm and a pulse width of 250 ps.
Other few examples of manufacturers or development teams which have worked with this technology in the past and/or are still developing it, but in the field of the modulated techniques are Mesa Imaging (a Centre Suisse d'Electronique et de Microtechnique (CSEM) spin-off), which have designed and commercialized a TOF imaging camera; PMD Technologies (a Zentrum für Sensorsysteme (ZESS) spin-off) from the Siegen University in Germany, which, similarly to CSEM, have developed an array-based TOF imaging camera; Optrima (an ETRO (Department of Electronics and Informatics) and VUB (Vrije Universiteit Brussel) spin-off), which have also developed a further TOF array-based imaging camera; and Canesta Inc., a company which, since 2004, has developed sensing devices used in TOF imaging cameras such as the previously described.
However, these imaging systems have limitations related to their receiving system, where the size of the light sensors comprised in the array of light sensors affects the resolution of the image of the scanned surface. Basically, given that the overall size of the light sensors (since it is difficult to integrate a sensor and its circuitry in one silicon chip using normal microelectronic schemes) is large, the number of portions of the light beam detectable by the scanning system is low (i.e. fewer portions can be distinguished), and therefore the spatial resolution of the scanned image is low. Since said spatial resolution of the digital image may be crucial when using the device in fields such as video generation techniques involving depth information in video images, guidance of vehicles or robots by means of 3D vision, quality control in manufacturing processes, etc. . . . , the existing systems, with lower spatial resolutions, may not be suitable to be used in said applications, if a high spatial resolution or performance of an existing device is required.
In summary, the known receiving systems have the drawback that the number of portions of the light beam received by the array of light sensors is too small, that is, from a given light beam and due to the size of the elements in the array, the amount of portions of the light beam that the systems can receive is too small (given that only one portion of the light beam is received by each light sensor), so spatial resolution of the scanned image is low. This way, the use of these receiving systems is limited to certain applications and even for these applications the results are not as good as one would wish.